Secondary batteries, such as lithium-ion secondary batteries, have been used in a wide variety of fields like cellular phones and notebook-size personal computers, because they are compact and have large capacities. A lithium-ion secondary battery has active materials, which can insert lithium (Li) thereinto and eliminate it therefrom, for the positive electrode and negative electrode, respectively. And, it operates because the Li ions migrate within an electrolytic solution that is disposed between both the electrodes.
The performance of secondary battery is dependent on materials for the positive electrode and negative electrode as well as the electrolyte that constitute the secondary battery. Even among them, the research and development of active-material ingredients that form active materials have been carried out actively. For example, employing silicon oxide (e.g., SiOx: “x” is 0.5≦“x”≦1.5 approximately) has been investigated. It has been known that SiOx decomposes into Si and SiO2 when being heat treated. This is called a disproportionation reaction; when being homogenous solid silicon monoxide, SiO, in which the ratio between Si and O is 1:1 roughly, it separates into two phases, an Si phase and SiO2 phase, due to the internal reaction in the solid. The Si phase, which is obtainable by the separation, is fine extremely. Moreover, the SiO2 phase, which covers the Si phase, possesses an action of inhibiting electrolytic solutions from decomposing. Therefore, a secondary battery, which uses a negative-electrode active material being made by decomposing SiOx into Si and SiO2, is good in terms of cyclability.
As a specific example for such a negative-electrode active material, a nano composite body, which includes Si, SiO2 and a metallic oxide, is disclosed in Patent Literature No. 1. In Example No. 3, a composite body with Si:SiO2:CaO=9:3:1 is obtained by heat treating SiO (e.g., a mixture with Si:SiO2=1:1) and calcium (Ca) at 900° C. for 6 hours in an argon gas atmosphere after mixing them in a molar ratio of 2:1.
Moreover, in Example No. 1 of Patent Literature No. 2, a composite body, in which B is doped into SiO1.48, is obtained by cooling SiO2, Si and B2O3 rapidly after subjecting them to a depressurized heat treatment at 800° C.